A conventional method of measuring the attenuation characteristics of such a fiber involves cutting the fiber into several sections of different length, transmitting a light pulse of given intensity to each section and measuring the intensity of the luminous radiation received at the exit end of each section; from the difference of the measured values, the mean attenuation coefficient can be determined. This technique, of course, cannot be utilized with fibers already installed in a light-transmission system.
A more expeditious method has been described by M. K. Barnoski and S. M. Jensen in a paper entitled "Fiber Waveguides: A Novel Technique For Investigating Attenuation Characteristics", published in APPLIED OPTICS, Vol. 15, No. 9, Sept. 1976. According to that proposal, radiant energy back-scattered from an optical fiber irradiated with pulsed light is intercepted and measured. The intensity and the wave shape of the back-scattered radiation supply information not only about the overall attenuation but also about the location of possible faults along the fiber.
In order to prevent spurious reflections from reaching the evaluation system, thereby falsifying the results, the article proposes to apply the luminous test pulse to a tapered zone of the fiber rather than to an end face thereof and to collect the back-scattered rays at an end of that zone. Such a procedure requires a delicate pretreatment of the fiber, with partial removal of any external coating present thereon.